1. Field of the Invention
The present invention relates to an electromagnetic pickup for an electric stringed instrument, e.g., an electric guitar or an electric piano, which converts a vibration signal to an electric signal by detecting the vibration of a string, and more particularly to an electromagnetic pickup for an electric stringed instrument that enables the great reduction in undesired noise, e.g., an electromagnetic noise, an electrostatic noise, an induced noise, etc.
2. Description of the Related Art
Generally, an electric stringed instrument, e.g., an electric guitar or an electric piano, has plural strings and a pickup for converting the vibration of a string to an electrical signal. There are two well-known types of pickups. One is an electromagnetic type of pickup and the other is a piezoelectric type of pickup. Particularly, in the case of an electric guitar, the electromagnetic type of pickup has been used since the introduction of the electric guitar, because by using the electromagnetic type of pickup it is easy to mount the pickup on the body of an electric guitar, to obtain a sound characteristic that emphasizes a medium sound region, and to process the resultant electrical signal.
The conventional configuration of a so-called "single coil type pickup" consists of a plurality of pole pieces each made of a cylindrical magnetic body, i.e., a permanent magnet, and respectively disposed corresponding to each string; a coil wound around the plurality of pole pieces; a bobbin on which the coil is wound; and a cover. The operation principle of a single coil type pickup is as follows. The pole pieces always emit magnetic flux from their top sections into a space above the pickup. When the strings vibrate in the space, the amount of the magnetic flux is changed, thereby inducing an electromotive force in response to the vibration of the strings, according to Fleming's right hand rule.
Arranging an electromagnetic pickup as described above, sharp and clear sound characteristics can be realized. Thus, a single coil type pickup is currently used. However, the configuration of a single coil type pickup is nearly the same as that of an antenna. Therefore, there is a problem in that such a single coil type pickup easily detects induced extraneous noise in addition to a normal vibration signal of strings, and thereby undesired noise is caused. Such induced noise that is roughly grouped into electromagnetic noise and electrostatic noise caused by an induced electromagnetic waves emitted from, e.g., an illumination device, a transformer, a vacuum tube, a radio receiver, etc. In particular, if a noise source exists in the direction of the axis of the pole pieces of the pickup, the induced noise has a significantly effect on an electromagnetic pickup.
To solve the problem described above, a humbucking type electromagnetic pickup disclosed in U.S. Pat. No. 2,894,491, was proposed by Seth E. Lover. This type of electromagnetic pickup is a so-called "double coil type pickup" or "humbucking pickup" that is basically formed as a parallel arrangement of two single coil type pickups. As described above, the two single coil type pickups respectively detect a vibration signal of a string and induced extraneous noise. However, since the pair of coils in this type of pickup are connected in reverse phase, induced noise is essentially cancelled out. Further, since the two rows of pole pieces in pickup have reverse magnetic poles from each other, electromotive force produced by the string vibration is added to and thereby increases.
In a humbucking pickup, induced noise is reduced as described above, and further, a powerful and sustained sound is obtained in comparison with a single coil type pickup. However, a humbucking pickup uses two single coil type pickups and detects the vibration of a string at two locations corresponding to the two single coil type pickups. Thereby, the two electromotive forces produced in coils of the pickups differ subtly from each other. Consequently, the waveform and phase of the overall output signal is distorted by the summing of the two electromotive forces and, in particular, the string vibration components consisting of harmonic overtones are decreased. Therefore, there is a problem in that a humbucking pickup outputs less clear and lower frequency sounds than a single coil type pickup. Further, there is a problem in that induced noises caused in the two single coil type pickups cannot completely cancel each other, because the pickups independently detect the vibration of a string at different points. Furthermore, there is a problem in that a humbucking pickup requires many components and thus is larger and more expensive. Therefore, the number of humbucking pickups that can be mounted on the body of a guitar is limited.
On the other hand, another type of single coil type electromagnetic pickup was disclosed in U.S. Pat. No. 3,236,930 by Clarence L. Fender. This electromagnetic pickup was developed for the purpose of reduction of induced noise. In this electromagnetic pickup, the bottom and side sections of the pickup, excluding the top section closest to the strings, are covered with permeability plates in order to reduce induced noise entering from the bottom and side directions, and further, to form a closed loop path conducting magnetic flux emitted from a pole piece and thereby reduce the effect of induced noise entering from the top direction.
However, it is not enough to reduce the effect of induced noise entering obliquely from the top direction, but it is also necessary to reduce the effect of induced noise entering from the axis direction of a pole piece.